It is relatively easy to measure the radiation spectrum from a target surface. The spectrum itself, however, does not yield information about the temperature without knowledge or information about the emissivity of the surface as a function of wavelength and possibly angle. The emissivity is often assumed to be constant (i.e., the gray body assumption) or to have a prescribed variation with wavelength. However, in actuality, the emissivity can vary strongly and without regularity as a function of wavelength. Hence, a lack of knowledge of the emissivity leads to a large uncertainty in the temperature obtained from the radiation measurement. In some present applications, the emitted light is compared to a surface having a known temperature, however, the validity of this comparison hinges on the assumption that the surface being measured and the reference temperature have the same emissivity.